1. Field of the Invention
The invention relates to a plastic molded pipe, in particular for fuel-conducting return lines in injection systems for internal combustion engines.
2. Description of the Prior Art
In fuel injection systems, such as unit injection systems (UI) or high-pressure reservoir injection systems (common rails), along with a high-pressure pump, high-pressure reservoir body, such as unit fuel injectors, and the fuel injectors in the return region for the fuel, that is, on the low-pressure side of the injection system, plastic pipes are used. By way of these molded plastic pipes disposed in the low-pressure region of the fuel injection system the fuel, such as a diverted control quantity, that occurs upon actuation of the fuel injectors is returned to the tank inside the fuel system.
The pipes used inside the low-pressure region are generally pipes formed in advance from plastic, which because of the installation conditions often have a shape that is other than rectilinear and are curved in arclike form. Molded plastic pipes can have a shape other than rectilinear for their internal contour and can have an external contour with a geometrically different course that is independent of the internal contour. Molded plastic pipes that carry fuel must meet stringent demands for strength and furthermore must have a high degree of security against bursting in the event of an accident. The molded plastic pipes used in the fuel system in the return region on the low-pressure region can be manufactured with and without throttle restrictions, which depends on the pressure to be preserved in the low-pressure region. In terms of their internal contour, molded plastic pipes that have a curve or an offset-bent portion or the like either cannot be shaped, or can be shaped only with relatively great effort. It is true that an arclike curved internal contour can be shaped, but no corner radius can be attained; moreover, freedom from burrs is not assured. In fuel-carrying molded plastic pipes that are in the crash zone and as a rule are made from high-strength plastic, there is a risk of fracture that increases with decreasing temperatures, so that in the low outdoor temperature range, there is the danger that this kind of molded plastic pipe will burst in the event of an accident and fuel will escape into the environment.
For this reason, in return pipes used in mass production in fuel injection systems, a special steel pipe extrusion-coated with a plastic jacket has been used. It is made by very complex shaping methods. Very high tolerance requirements must be met in order to attain the connection geometries with regard to the injection molding tool, the at least one fuel injector to be connected, and the connection layout specified by the automobile manufacturer. In a variant of the plastic pipe in the low-pressure region of the fuel injection system, inside which a remaining residual pressure level must be maintained, a bush throttle restriction must be fixed inside the inserted special steel pipe; in general, this is attained by roll-indenting the special steel and fixing the throttle restriction, which is preferably embodied as a bush throttle restriction, in the pipe by means of the two axially spaced-apart recesses created by the roll-indenting process.
The embodiment of the molded plastic pipe with an integrated special steel pipe, known from the prior art and used in large-scale mass production, has a number of disadvantages for the above-mentioned reasons, such as high weight, complicated shaping processes, special work steps for fixing possible throttle restrictions, and the like, so that a remedy must be created here.